There are several kinds of polymorphs in boron nitride They are hexagonal boron nitride (hereinafter h-BN), rhombohedral boron nitride (hereinafter r-BN), amorphous boron nitride (hereinafter a-BN), turbostratic boron nitride (hereinafter t-BN), zinc blend type cubic boron nitride (hereinafter c-BN), and wurtzite type hexagonal boron nitride (hereinafter w-BN), the latter two being high pressure stable phases. In case where boron nitride is synthesized at relatively low temperatures, for example at 900.degree. C. or below, a-BN is generated. It is known that if boron nitride synthesis is carried out at elevated temperature or if a-BN is heat treated at elevated temperature, h-BN is generated. The primary particles of h-BN powder are usually hexagonal platelets.
FIG. 1 is X-ray powder diffraction diagram of h-BN which exhibits remarkable [002], [100], [101], [102] and [004] diffraction spectra. Compared with this h-BN diffraction diagram, the diffraction diagram of a-BN generally exhibits a broad diffraction peak having a large half peak height width at the site where the [002], [100] and [101] diffraction peaks of h-BN are present. As shown in FIG. 8 the broad diffraction peak seems to be a composite of the [100] and [101] diffraction peaks of h-BN (hereinafter, for convenience of explanation, every diffraction peaks of boron nitride other than h-BN in respective X-ray powder diffraction diagrams are simply named with indexes of h-BN without notices). The crystal of h-BN has a crystal structure having hexagonal B-N net layers stacked up in a pattern of a,a', a,a', a,a', a,a', . . . With other typed patterns of stacked hexagonal B-N mesh layers, other kinds of crystal phases other than h-BN will result, and a crystal phase having a random manner of stacked hexagonal B-N mesh layers is generally called t-BN.
In a wide sense, as explained in "Shigen-Sozai-Gakkai-Shi (J. Resources and Materials Association) " Vol. 105 No. 2, p 201, a-BN may be classified as one sort of BN having a turbostratic structure. However, a boron nitride having such broad X-ray diffraction peaks should be amorphous BN, i.e., a-BN. In the present invention, a well crystallized boron nitride, having sharp and remarkable [004] and other diffraction peaks in the X-ray powder diffraction diagram, is termed "t-BN". (Particularly, as explained later, the present invention intends a crystal phase having a specified small half peak height width of the [004] spectra 2.theta. and having no ordering (pattern) in the laminating manner, as the crystalline t-BN).
Boron nitride crystals have crystal structures similar to graphite and exhibit properties similar to graphite crystals except for electrical insulating property. For example, boron nitride crystals have weak bonding strength between each hexagonal B-N mesh layers, and are easily cleaved into flakes to exhibit solid lubricity. Boron nitride is stable up to high temperatures under a nonoxidizing atmosphere, hard to sinter, with its sintered body having good machinability. Yet more, it is not oxidized up to about 1000 .degree. C., which is by about 500 .degree. C. higher than the oxidation resistant temperature of graphite. Boron nitride has attractive properties as a material. If boron nitride powder having superior sinterablity or high purity is produced cheaply and sintered bodies can be supplied in low cost, boron nitride will find many new applications which are not economically feasible at present.